Neuroschistosomiasis is an infection of the nervous system by a blood fluke of the genus Schistosoma. A common infection within tropical regions, neuroschistosomiasis can cause substantial neurologic disability. In this updated article, the author discusses the role of humans and snails in the fascinating life cycle of schistosomal worms, the presumed pathophysiology of neuroschistosomiasis, and advances in attempts to reduce this parasitic infection through vaccination and snail population controls.

Key points

• Schistosomiasis is a common intravascular infection caused by parasitic Schistosoma trematode worms.

• People are infected during routine agricultural, domestic, occupational, and recreational activities that expose them to infested water.

Humans have been afflicted with schistosomiasis for more than 2500 years (El-Khoby et al 2000). However, it was not until 1851 that Theodor Bilharz Maximilian, a German surgeon, discovered the trematode worm in an autopsy while working at the Kasr-el-Aini Hospital of Cairo in Egypt, initially naming it Distomum haematobium. Later in 1851, he communicated his findings to his former teacher, Carl Theodor Ernst von Siebold in a series of letters. In 1853, extracts from these letters together with von Siebold's comments were published in the German Zoological Journal (Bilharz and von Siebold 1853).

Due to the peculiar morphology of the worm, it was clear that it could not be included in the genus Distomum as was initially proposed; thus, the parasite was described in 1856 as Bilharzia haematobium by Meckel Von Hemsbach in a thesis entitled “The Geology of the Human Body” (Meckel 1856). Weinland, apparently not knowing of this thesis, re-described the worm as Schistosoma haematobium in 1858 (Weinland 1858). However, in 1948, the International Commission on Zoological Nomenclature established the name Schistosoma and it is, thus, the current name of the parasite (International Commission on Zoological Nomenclature 1950). Bilharz also described Schistosoma mansoni, but this species was re-described by Louis Westenra Sambon in 1907 at the London School of Tropical Medicine, who named it after his teacher Patrick Manson (Brant et al 2006). The life cycle was described by da Silva in 1908 (Brant et al 2006).

Molecular phylogenetic studies suggest that Schistosoma spp. originated in Asia and that a pulmonate-transmitted progenitor colonized Africa and gave rise to both the terminal-spined and lateral-spined egg species groups, the latter containing Schistosoma mansoni (Sambon 1907).

Schistosoma mansoni likely appeared only after the transatlantic dispersal of Biomphalaria from the neotropica to Africa, an event based on African fossil record, occurred about 2 to 5 million years ago. This parasite became abundant in tropical Africa and, much later, entered the New World with the slave trade (Morgan et al 2003).

Schistosomiasis was originally thought to have been exported from Africa to South America during the period of slave trade where it found favorable climatic conditions that promoted its spread. Factors that favor spread include growth in international travel, refugee and population migration, and the development of new water resources (Ross at el 2002).

Five species of Schistosoma to infect humans: Schistosoma mansoni, Schistosoma hematobium, and Schistosoma japonicum are the most widely distributed and are present in multiple tropical and subtropical countries whereas Schistosoma mekongi and Schistosoma intercalatum are much more restricted in their geographic distributions. Most granulomas following schistosomal infection develop in the intestine and liver (Schistosoma mansoni and Schistosoma japonicum), or genitourinary tract (Schistosoma haematobium) (Ross et al 2002). Cerebral schistosomiasis is usually caused by Schistosoma japonicum whereas myelopathy is most often induced by Schistosoma mansoni and Schistosoma haematobium (Pollner 1994).

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